1. Field of Invention
The invention relates to a motor driver suited to driving a brushless DC motor, etc. used in, for example, air-conditioning equipment, water heaters (on which a fan motor for burning is mounted) air cleaners, and information-processing equipment, such as copying machines, printers, etc. In particular, the invention relates to a motor driver for driving a motor in a sinewave driving system capable of considerably reducing torque ripple, vibration, and noise at the time of motor driving.
2. Description of the Related Art
Brushless DC motors (referred below to as motor) are in many cases used in various driving motors of electrical equipment such as air-conditioning equipment, etc., while merits thereof, such as long life, high reliability, and easiness in speed control are made the most of.
Conventionally, a rectangular-wave driving system, in which a drive voltage supplied to a coil has a rectangular-wave shaped drive waveform, has been widely adopted as a motor driving system. In recent years, however, a request has been increasingly made for driving a motor, such that the motor has low torque ripple, low vibration, and low noise. A sinewave driving system, in which drive voltage supplied to a coil of a motor has a sinewave shaped drive waveform, becomes general as a driving system to meet the above-noted request.
A technique for sinewave driving of a motor is conventionally described in, for example, Japanese Patent Gazette No. 3232467. This conventional motor driver sequentially reads sinewave shaped waveform data stored in a memory according to a rotational position of a motor. The motor driver modulates the waveform data in pulse width to control respective switching elements of an inverter circuit, which supplies electric power to a coil of a motor. In this manner, the conventional motor driver sinewave-drives a motor.
A further technique is described in, for example, Japanese Patent Unexamined Publication No. 2003-348874. This conventional motor driver realizes a sinewave drive technique by using a semiconductor integrated circuit to reduce the number of parts and cost.
FIG. 5 is a circuit structure diagram of a conventional motor driver of this kind. FIG. 6 is a view illustrating an operation of the motor driver shown in FIG. 5.
In FIG. 5, motor 130 includes a moving body (not shown) and three-phase coil 131. Drive voltage and drive current are supplied to coil 131 through a plurality of switching elements provided on energizing unit 120 from a dc power source (not shown).
The motor driver includes controller 110, which includes waveforms generator 114, energizing unit 120, and speed-position detector 140.
Respective information of position and speed of the moving body of the motor are detected by speed-position detector 140 to be transmitted, as output signal RS, to waveforms generator 114 in controller 110. Waveforms generator 114 outputs sinewave-shaped control signals UG, VG, and WG on the basis of the transmitted respective information of position and speed of the moving body. Energizing unit 120 uses control signals UG, VG, and WG to make the respective switching elements ON or OFF, thereby applying drive voltages U, V, and W to coil 131 to supply drive currents Iu, Iv, and Iw thereto.
Control signals UG, VG, and WG are ones having phase difference of electrical angle 120 degrees relative to one another. In order to make the switching elements in energizing unit 120 ON or OFF, such control signals are not limited to three signals but are output as six signals in some cases. As such control signals, signals being modulated in pulse width are used in many cases.
FIG. 6 shows waves of a state of a U-phase coil, that is, output signal RS being an output of speed-position detector 140, drive voltage U and drive current Iu being outputs of energizing unit 120, and induced voltage Eu of the coil.
Waveforms generator 114, into which output signal RS of speed-position detector 140 is input, uses drive-speed information and position information of the moving body, which are included in output signal RS, to output sinewave-shaped drive voltage U. Drive voltage U, induced voltage Eu, and drive current Iu determined by impedance of the coil are supplied from energizing unit 120.
Output signal RS is not necessarily required to be put in a phase relationship, as shown, with drive voltage U and induced voltage Eu, but suffices to be a signal including drive-speed information and position information instead of being rectangular-wave shaped.
While not shown, the relationship among drive voltage, induced voltage, and drive current for V-phase and W-phase is the same as described above.
Motor 130 is sinewave-driven in the above manner.
However, the conventional motor driver described above involves a problem that it is not possible to highly efficiently drive motor 130.
In order to highly efficiently drive a motor, it is necessary to make a phase of drive current, which flows through a coil, and a phase of induced voltage in agreement with each other.
Drive current flowing through a coil assumes a value obtained by dividing voltage, which is obtained by subtracting induced voltage from drive voltage applied to the coil, by impedance of the coil. Impedance of the coil includes inductance component. Therefore, the phase of drive current is behind the phase of drive voltage. Accordingly, in order to highly efficiently drive a motor, taking account of delay of the phase of drive current relative to drive voltage, it is necessary to advance the phase of drive voltage so that the phase of induced voltage and the phase of drive current agree with each other.
Drive voltage of the conventional motor driver shown in FIG. 5 is sinewave-shaped and sinewave is varied in period by drive speed but cannot be advanced in phase. Consequently, as shown in FIG. 6, the conventional motor driver involves a problem that it is not possible to make, for example, induced voltage Eu and drive current Iu in agreement with each other, and so the motor is lowered in drive efficiency.